A conventional metal detector includes an oscillator coil which generates a high frequency electromagnetic field within the region of a measuring zone commonly referred to as a detector head. The detector head is formed to include an enclosed volume having an entrance and an exit aperture. The detector head also contains the coil form on which the oscillator coil is wound. An article suspected of containing metal is passed through the detector head aperture by means of, for example, a conveyor. Ideally, all of the electromagnetic field remains within the enclosed volume of the detector head, but in practice some of the field escapes through the aperture. Also residing within the detector head are a pair of receiver coils, symmetrically spaced on either side of the oscillator coil and typically wound on the same coil form. The receiver coils are wired so as to be in series opposition. Through the positioning of the oscillator/receiver coil assembly and by other means well known to those skilled in the art, the signal induced by the oscillator onto each of the receiver coils is of the same magnitude but of opposite polarity. As a result, the net voltage output of the interconnected receiver coils is zero volts.
When metal passes through the detector head aperture, the electromagnetic field within the detector head is disturbed or distorted. The change in the electromagnetic field induces a difference in the signal received by each of the receiver coils. The net voltage sum of the two receiver coils is no longer zero. This perceived change or difference of signal in the receiver coils is then amplified and analyzed to determine if the characteristics of the distorted field are consistent with the presence of metal.
Similarly, the product being tested for the presence of metal also induces a difference in the signal produced by each of the receiver coils. Even though the product is nonmetallic, it will typically have properties such as permeability and conductance that will effect the behavior of an electromagnetic field. This effect itself may be small compared to the change due to metal, but the volume of the product is much greater than that of the metal to be detected. This effect may result in a product signal that is many times greater than the signal from the metal to be detected. Phase sensitive signal analysis is typically used to extract the metal signal from the product signal.
Since the electromagnetic field also protrudes outside of the detector aperture, the presence of metal (and to a lesser extent other materials, such as other products about to be tested) in such exterior regions will also distort the electromagnetic field outside as well as inside the detector head. If the metal or product residing outside of the detector head is in motion, this typically vibrating type of motion creates a time varying change in the electromagnetic field.
Such a change in the electromagnetic field appears as a change or difference in the signal produced by each of the receiver coils and is thus detected as moving metal. In the state of the art metal detector, moving metal outside of the aperture is indistinguishable from metal passing through the aperture. Such a situation can result in the metal detector incorrectly rejecting a metal free article or improperly activating an alarm, either of which results in a loss of production.
A possible solution to the problem of detecting metal outside of the detector head aperture is to decrease the sensitivity of the of the receiver coils, which results in a larger minimum size of detectable metal. Another solution is to track and thereby learn the moving metal characteristics, which may also result in a loss of sensitivity to metal passing through the aperture. A final solution is to attempt to better contain the electromagnetic field entirely within the detector head, which results in a much higher cost of the electronics or the detector head.
An example of such a metal detector is disclosed in U.S. Pat. No. 5,572,121, which addresses the problem of creating a virtual metal free zone by generating primary and secondary electromagnetic fields in opposition to each other. The interaction of the secondary opposing field with the primary field limits the distance or range that the primary field extends from the detector head and creates a barrier to further extension of the primary field. This prevents the primary field from being influenced by the presence of metal beyond that finite distance.
The aforementioned technique is not entirely satisfactory. In order to contain the primary field, a field of the opposite polarity is generated which itseld extends outside of the detector head. The new field is also sensed by the detector coils as was the primary field, only with the opposite polarity. Shaping the field using metal forms and shapes is also relatively ineffective because the resonant frequency of the forms is so high when compared to the relatively long wavelengths associated with the oscillator frequency.